Modern commercial aircraft have horizontal stabilizers that pivot relative to the airplane fuselage to "trim" the aircraft during flight, i.e., adjust the position of the horizontal stabilizer to accommodate for different load distributions within the aircraft. One common horizontal stabilizer trim actuator consists of a ball nut mounted in a gimbal at the leading edge of the center of the horizontal stabilizer structure, and an upright ballscrew extending through the ball nut. The ballscrew, in turn, has its bottom end mounted in a lower gimbal secured to the fuselage. By rotating the ballscrew in one direction, the leading edge of the horizontal stabilizer is moved up, whereas by rotating the ballscrew in the other direction, the leading edge of the horizontal stabilizer is moved down. Rotation of the ballscrew can be by an electric motor and associated gearing.
The horizontal stabilizer hinge moment is transmitted through the stabilizer gimbal and ball nut to the ballscrew. This load has a vertical component as well as a torque component due to ballscrew lead (threads). Depending on whether the stabilizer is nose up or nose down, the vertical load may be either compressive or tensile. To prevent freewheeling of the ballscrew due to the aerodynamic load on the horizontal stabilizer, some type of brake must be provided, but the brake must not prevent all rotation of the ballscrew because trimming in either direction (against or with the aerodynamic induced torque load) may be desirable. These types of brakes are referred to as "no-back" devices, and desirable characteristics are: for a given load, the input torque required is the same for lifting or lowering the load; the device must prevent backdriving by the aerodynamic load with a considerable safety margin; the brake torque variability due to wear, heat, variations in material properties, and manufacturing variability should be as small as possible; the braking force should be consistent, i.e., resistant to brake fade over a long operating life and despite frequent use; and the device should be compact, lightweight and use readily available materials.